U.S. patent number 10,122,489 [Application Number 14/666,527] was granted by the patent office on 2018-11-06 for polarization state detector, optical transmitter, and optical receiver.
This patent grant is currently assigned to FUJITSU LIMITED. The grantee listed for this patent is FUJITSU LIMITED. Invention is credited to Hisao Nakashima.
United States Patent |
10,122,489 |
Nakashima |
November 6, 2018 |
Polarization state detector, optical transmitter, and optical
receiver
Abstract
An optical transmitter transmits optical signal including a
first signal and a second signal. The second signal is subjected to
change in a polarization state relative to a polarization state of
the first signal. An optical receiver analyzes a reception
characteristic of the second signal and detects, based on the
analyzed result, a polarization state of the first signal
indicative of a higher signal quality than that of the second
signal.
Inventors: |
Nakashima; Hisao (Kawasaki,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
FUJITSU LIMITED |
Kawasaki-shi, Kanagawa |
N/A |
JP |
|
|
Assignee: |
FUJITSU LIMITED (Kawasaki,
JP)
|
Family
ID: |
52780392 |
Appl.
No.: |
14/666,527 |
Filed: |
March 24, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150280856 A1 |
Oct 1, 2015 |
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Foreign Application Priority Data
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Mar 26, 2014 [JP] |
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2014-063951 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J
14/06 (20130101); H04B 10/6162 (20130101); H04B
10/0779 (20130101); H04B 10/614 (20130101); H04B
10/2572 (20130101) |
Current International
Class: |
H04J
14/06 (20060101); H04B 10/61 (20130101); H04B
10/077 (20130101); H04B 10/2507 (20130101) |
Field of
Search: |
;398/152,65 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2690801 |
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Jan 2014 |
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EP |
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2009-089194 |
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Apr 2009 |
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JP |
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2009-133840 |
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Jun 2009 |
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JP |
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2010-109705 |
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May 2010 |
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JP |
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2010-226499 |
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Oct 2010 |
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JP |
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2013/114629 |
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Aug 2013 |
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WO |
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Other References
"Know." The Chambers Dictionary, C.M. Schwarz, Chambers Harrap,
13th edition, 2015. Credo Reference,
http://search.credoreference.com/content/entry/chambdict/know/0?instituti-
onId=743. Accessed Oct. 4, 2017. cited by examiner .
EESR--Extended European Search Report dated Jul. 29, 2015 for
corresponding European Patent Application No. 15159911.5. cited by
applicant .
EPOA--European Office Action dated Mar. 14, 2017 for corresponding
European Patent Application No. 15159911.5. cited by applicant
.
JPOA--Office Action of Japanese Patent Application No. 2014-063951
dated Nov. 21, 2017, with relevant machine translation of the
office action. cited by applicant .
EPOA--Office Action of European Patent Application No. 15 159 911.5
dated Mar. 16, 2018. cited by applicant.
|
Primary Examiner: Payne; David
Assistant Examiner: Motsinger; Tanya
Attorney, Agent or Firm: Fujitsu Patent Center
Claims
What is claimed is:
1. A polarization state detector comprising: an analyzer configured
to receive an optical signal including a transmission data signal
and a monitor signal from an optical transmitter, the monitor
signal being periodically transmitted between transmissions of a
plurality of the transmission data signals, being controlled in the
optical transmitter to be changed in a polarization state relative
to a polarization state of the transmission data signal, and being
a known signal between the polarization state detector and the
optical transmitter, and to analyze a reception signal quality of
the monitor signal for each different polarization state of the
monitor signal; and a detector configured to detect, based on the
analyzed result of the analyzer, the polarization state of the
transmission data signal corresponding to the polarization state of
the monitor signal indicative of a relatively higher reception
signal quality among the plurality of the reception signal quality
for the different polarization states, wherein the transmission
data signal and the monitor signal are modulated by using mutually
different modulation schemes, wherein a dummy signal modulated by
using the same modulation scheme as that of the transmission data
signal is transmitted by the optical transmitter between the
transmission data signal and the monitor signal, and wherein the
dummy signal is a dummy of the transmission data signal and is
transmitted in a guard period provided between transmissions of the
transmission data signal and the monitor signal.
2. The polarization state detector according to claim 1, wherein
the monitor signal is modulated with a
Stokes-vector-modulation.
3. An optical transmitter comprising: a signal processor configured
to generate a transmission data signal and a monitor signal, the
monitor signal being periodically transmitted between transmissions
of a plurality of transmission data signals, being controlled in
the optical transmitter to be changed in a polarization state
relative to a polarization state of the transmission data signal,
and being a known signal between a polarization state detector and
the optical transmitter; and an optical modulator configured to
modulate transmission light based on the transmission data signal
and the monitor signal to generate transmission optical signals to
be transmitted to the polarization state detector, wherein the
signal processor is configured to modulate the transmission data
signal and the monitor signal by using mutually different
modulation schemes and to output a dummy signal modulated by using
the same modulation scheme as that of the transmission data signal
between the transmission data signal and the monitor signal.
4. The optical transmitter according to claim 3, wherein the signal
processor includes a Stokes-vector-modulator configured to generate
the monitor signal by using a Stokes-vector-modulation.
5. The optical transmitter according to claim 3, wherein the signal
processor includes a polarization controller configured to receive
the polarization state of the transmission data signal notified
from an optical receiver and to control the polarization state of a
transmission data signal to be transmitted based on the notified
polarization state detected by the optical receiver that receives
the transmission optical signals, and wherein the notified
polarization state corresponds to the polarization state of the
monitor signal at the optical receiver and is indicative of a
relatively higher reception signal quality among a plurality of
reception signal quality at the optical receiver for different
polarization states caused by a polarization control performed by
the polarization controller in the optical transmitter.
6. An optical receiver comprising: an analyzer configured to
receive a transmission optical signal including a transmission data
signal and a monitor signal transmitted from an optical transmitter
and to analyze a reception signal quality of the monitor signal for
each different polarization state of the monitor signal, the
transmission data signal and the monitor signal, which are
controlled in the optical transmitter, being generated by the
optical transmitter, the monitor signal being periodically
transmitted between transmissions of a plurality of the
transmission data signals and being changed in a polarization state
relative to a polarization state of the transmission data signal,
and the transmission optical signal being generated by modulating
transmission light based on the transmission data signal and the
monitor signal by the optical transmitter; and a detector
configured to detect, based on an analyzed result of the analyzer,
the polarization state of the transmission data signal
corresponding to the polarization state of the monitor signal
indicative of a relatively higher reception signal quality among
the plurality of the reception signal quality for the different
polarization states, wherein the transmission data signal and the
monitor signal are modulated by using mutually different modulation
schemes, wherein a dummy signal modulated by using the same
modulation scheme as that of the transmission data signal is
transmitted by the optical transmitter between the transmission
data signal and the monitor signal, and wherein the dummy signal is
a dummy of the transmission data signal and is transmitted in a
guard period provided between transmissions of the transmission
data signal and the monitor signal.
7. The optical receiver according to claim 6, wherein the optical
transmitter is notified of the polarization state detected by the
detector by using supervisory light transmitted to the optical
transmitter.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of priority
of the prior Japanese Patent Application No. 2014-063951, filed on
Mar. 26, 2014, the entire contents of which are incorporated herein
by reference.
FIELD
The embodiment discussed herein is related to a polarization state
detector, a method of detecting a polarization state, an optical
communication system, an optical transmitter, and an optical
receiver.
BACKGROUND
As examples of technology to realize high-speed and high-capacity
optical transmission, a polarization multiplexing technology and a
multi-value modulation technology are known. For example, the
polarization multiplexing technology transmits signals assigned or
mapped to two orthogonal polarized waves. Meanwhile, the
multi-value modulation technology modulates light to be transmitted
by using a modulation scheme available to transmit information of
multiple bits in one symbol time. The modulation scheme may be a
phase-shift keying including QPSK or a quadrature amplitude
modulation of 2.sup.M-QAM (where, M represents the number of
multi-levels and is an integer of two or more).
In addition, a digital coherent reception technology using digital
signal processing may be applied to a reception side of an optical
signal. In the digital coherent reception technology, a received
signal light is mixed with a local oscillation light of a local
oscillator source by 90 degrees optical hybrid. Thereby,
information indicative of the amplitude and the phase of the
optical signal with reference to the local oscillator light is
extracted. The extracted information (signal) is digitalized by an
analog-to-digital converter (ADC), and the digital signal is
demodulated by using a digital signal processing. The digital
signal processing is possible to compensate a waveform distortion
(in other words, degradation of a signal quality) of received
signal light due to wavelength dispersion, polarization mode
dispersion, and the like of the optical transmission line.
In contrast, since a polarization state of a transmitted optical
signal varies with time, it is difficult to compensate for
degradation of the signal quality due to a polarization dependence
loss (PDL), which occurs in an optical transmission line, an
optical repeater, or the like, by using digital signal processing
on the reception side. For this reason, the PDL is a major factor
that limits the transmission capability of an optical signal.
In order to absorb the degradation of the signal quality due to the
PDL, a scheme to average the polarization states of a transmitted
optical signal (or a scheme corresponding to an averaging process)
has been proposed (for example, see JP 2009-89194 A and JP
2010-109705 A).
JP 2009-89194 A discloses that data to be transmitted with two
orthogonal polarizations are interleaved. Thus, according to JP
2009-89194 A, even if the PDL is present in an optical transmission
line, an optical repeater, or the like, it is possible to average
bit error rates (BERs) between the polarizations.
Meanwhile, JP 2010-109705 A discloses a high-speed polarization
scrambling process achieved by a polarization scrambling process
with digital signal processing. Thus, according to JP 2010-109705
A, similar to JP 2009-89194 A, even if the PDL is present in an
optical transmission line, an optical repeater, or the like, it is
also possible to average the BERs between the polarizations.
As examples of technology to monitor the PDL, technologies
disclosed in JP 2009-133840 A and JP 2010-226499 A are known.
However, according to the technologies to average the polarization
states of a transmitted optical signal by using a scrambling
process, as disclosed in JP 2009-89194 A and JP 2010-109705 A, a
maximum penalty due to the PDL of the transmitted optical signal
may be absorbed but the averaging process merely achieves a small
improvement in the BERs. Meanwhile, in order to further improve in
the BER, the varying of the polarizations may be speeded-up on the
transmission side in the technology disclosed in JP 2010-109705 A.
However, the reception side may be unavailable to follow (or track)
a change in the polarizations. Hence, the influence for the optical
signal on the reception characteristic is large.
Both of JP 2009-133840 A and JP 2010-226499 A merely disclose
technologies to measure (or monitor) the PDL of an optical
transmission line.
SUMMARY
One aspect of a polarization state detector may include an analyzer
and a detector. The analyzer receives an optical signal including a
first signal and a second signal from an optical transmitter. The
second signal is subjected to change in a polarization state
relative to a polarization state of the first signal. Further, the
analyzer analyzes a reception characteristic of the second signal.
The detector detects, based on the analyzed result of the analyzer,
a polarization state of the first signal indicative of a higher
signal quality than that of the second signal.
The object and advantages of the invention will be realized and
attained by means of the elements and combinations particularly
pointed out in the claims.
It is to be understood that both the foregoing general description
and the following detailed description are exemplary and
explanatory and are not restrictive of the invention.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram that illustrates an exemplary
configuration of an optical communication system according to an
embodiment;
FIGS. 2A to 2C are diagrams that illustrate an influence of PDL on
a polarization-division-multiplexed signal light transmitted in the
optical communication system illustrated in FIG. 1;
FIG. 3 is a diagram that illustrates an example of a change in a Q
penalty with respect to an input polarization state (angle) for a
PDL medium;
FIG. 4 is a diagram that illustrates an example of a transmission
signal format used in the optical communication system illustrated
in FIG. 1;
FIG. 5 is a block diagram that illustrates an exemplary
configuration of a digital signal processor (transmission signal
processor) of an optical transmitter illustrated in FIG. 1;
FIG. 6 is a block diagram that illustrates an exemplary
configuration of a detection-oriented signal inserter illustrated
in FIG. 5;
FIG. 7 is a block diagram that illustrates an exemplary
configuration of a digital signal processor (reception signal
processor) of an optical receiver illustrated in FIG. 1;
FIG. 8 is a block diagram that illustrates an exemplary
configuration of the digital signal processor (transmission signal
processor) of the optical transmitter illustrated in FIG. 1;
FIG. 9 is a block diagram that illustrates a first modified example
of the transmission signal processor illustrated in FIG. 5;
FIG. 10 is a diagram that illustrates Stokes-vector-modulation
performed by the transmission signal processor for a
detection-oriented signal illustrated in FIG. 9;
FIG. 11 is a block diagram that illustrates an exemplary
configuration of a reception signal processor for the transmission
signal processor illustrated in FIG. 9;
FIG. 12 is a block diagram that illustrates a modified example of
the reception signal processor illustrated in FIG. 11; and
FIG. 13 is a diagram that illustrates an example of a transmission
signal format according to a second modified example of the
embodiment.
DESCRIPTION OF EMBODIMENTS
Hereinafter, an embodiment will be described with reference to the
drawings. However, the embodiment described below is merely an
example but is not intended to exclude various modifications or
technical applications unless otherwise specified in the following
embodiment. In the drawings used in the following embodiment, the
same reference numerals used in the embodiment denote the same
elements unless otherwise mentioned.
(Embodiment)
FIG. 1 is a block diagram that illustrates an exemplary
configuration of an optical communication system 1 according to an
embodiment. The optical communication system 1 illustrated in FIG.
1 is an example of an optical communication system with a digital
coherent transmission technology. The optical communication system
includes, for example, an optical transmitter 10 that transmits an
optical signal to an optical transmission line 50, and an optical
receiver 30 that receives the optical signal transmitted through
the optical transmission line 50. An optical fiber is applicable to
the optical transmission line 50. In addition, the optical
transmission line 50 may be provided with one or more of optical
repeaters including an optical amplifier according to a
transmission distance of an optical signal.
Although FIG. 1 illustrates an exemplary configuration of the
optical communication system 1 with focusing on unidirectional
optical transmission, the optical communication system 1 may be
configured to perform bidirectional optical communication. For
example, a first optical transmission apparatus provided with the
optical transmitter 10 may be connected to a second optical
transmission apparatus that is opposed to the first optical
transmission apparatus and is provided with the optical receiver 30
through a bidirectional optical transmission line 50.
In such a case, the first optical transmission apparatus may
include an optical receiver with a configuration equivalent to that
of the optical receiver 30 in the second optical transmission
apparatus as an example of a reception system. Meanwhile, the
second optical transmission apparatus may include an optical
transmitter with a configuration equivalent to that of the optical
transmitter 10 in the first optical transmission apparatus.
(Optical Transmitter)
The optical transmitter 10 includes, for example, a digital signal
processor (DSP) 11, a light source 12, four of digital-to-analog
converters (DACs) 13, four of drivers 14, two of IQ modulators 15X
and 15Y, and a polarization beam combiner (PBC) 16.
The digital signal processor 11 performs digital signal processing
on a transmission data signal that is an electrical signal. The
digital signal processor 11 may be achieved by using, for example,
a field programmable gate array (FPGA), a large-scale integration
circuit (LSI), or the like.
The digital signal processing may include polarization control for
two orthogonal polarizations X and Y and a digital modulation
process according to a modulation scheme such as phase-shift keying
(PSK) or quadrature amplitude modulation (QAM).
The digital modulation process is possible to generate, for
example, data XI and YI to be mapped to the two orthogonal
polarizations X and Y, respectively, as a in-phase component (I
component) and to generate data XQ and YQ to be mapped to the two
orthogonal polarizations X and Y, respectively, as a quadrature
components (Q).
In other words, the data XI and XQ respectively represent
I-component data and Q-component data to be mapped to one
polarization X, and the data YI and YQ respectively represent
I-component data and Q-component data to be mapped to the other
polarization Y.
The light source 12 generates light (for example, continuous light)
for transmission and inputs the generated light to the IQ
modulators 15X and 15Y.
The DACs 13 respectively convert the data XI, XQ, YI, and YQ input
from the digital signal processor 11 from digital signals into
analog signals and inputs the converted analog signals to
corresponding drivers 14. Among the four DACs 13, two DACs 13
correspond to the polarization X, and the other two DACs 13
correspond to the polarization Y.
One of the two DACs 13 corresponding to the polarization X converts
the I-component data XI to be mapped to the polarization X into an
analog signal, and the other DAC 13 thereof converts the
Q-component data XQ to be mapped to the polarization X into an
analog signal. Meanwhile, one of the two DACs 13 corresponding to
the polarization Y converts the I-component data YI to be mapped to
the polarization Y into an analog signal, and the other DAC 13
thereof converts the Q-component data YQ to be mapped to the
polarization Y into an analog signal.
The drivers 14 generate drive signals for the IQ modulators 15X and
15Y. For example, among the four drivers 14, two drivers 14
correspond to the polarization X, and the other two drivers 14
correspond to the polarization Y.
One of the two drivers 14 corresponding to the polarization X
generates a drive signal according to the I-component data XI
converted into the analog signal for the IQ modulator 15X
corresponding to the polarization X. Meanwhile, the other of the
two drivers 14 corresponding to the polarization X generates a
drive signal according to the Q-component data XQ converted into
the analog signal for the IQ modulator 15X corresponding to the
polarization X.
One of the two drivers 14 corresponding to the polarization Y
generates a drive signal according to the I-component data YI
converted into the analog signal for the IQ modulator 15Y
corresponding to the polarization Y. Meanwhile, the other of the
two drivers 14 corresponding to the polarization Y generates a
drive signal according to the Q-component data YQ converted into
the analog signal for the IQ modulator 15Y corresponding to the
polarization Y.
Here, the drive signal generated according to the I-component data
XI corresponding to the polarization X may be simply referred to as
a "drive signal XI", and the drive signal generated according to
the Q-component data XQ corresponding to the polarization X may be
simply referred to as a "drive signal XQ". Similarly, the drive
signals respectively generated according to the I-component data YI
and the Q-component data YQ corresponding to the polarization Y may
be simply referred to as a "drive signal YI" and a "drive signal
YQ".
Each of the IQ modulators 15X and 15Y may be an optical modulator
such as a Mach-Zehnder optical modulator and modulates the
continuous light input from the light source 12 with the drive
signals input from two of the drivers 14 to generate transmission
modulation signal light of the polarization X or transmission
modulation signal light of the polarization Y.
For example, the IQ modulator 15X corresponding to the polarization
X modulates the continuous light input from the light source 12
with the drive signals XI and XQ input from two of the drivers 14
to generate the transmission modulation signal light of the
polarization X.
Meanwhile, the IQ modulator 15Y corresponding to the polarization Y
modulates the continuous light input from the light source 12 with
the drive signals YI and YQ input from two of drivers 14 to
generate the transmission modulation signal light of the
polarization Y.
The PBC 16 combines (or multiplexes) the transmission modulation
signal light of the polarization X generated by one IQ modulator
15X and the transmission modulation signal light of the
polarization Y generated by the other IQ modulator 15Y.
Transmission modulation signal light (it may be referred to as
"polarization multiplexed signal light") generated by the PBC 16 is
transmitted to the optical transmission line 50.
Here, the light source 12, the driver 14, the IQ modulators 15X and
15Y, and the PBC 16 described above may be considered as an example
of an optical modulator that modulates transmission light input
from the light source 12 with transmission data and a
detection-oriented signal described later.
(Optical Receiver)
Meanwhile, the optical receiver 30 illustrated in FIG. 1 is an
optical receiver available to perform digital coherent reception.
Thus, the optical receiver 30 includes, for example, a local
oscillator light source 31, a polarization beam splitters (PBSs) 32
and 33, and two of 90-degrees hybrids 34. These elements may form
an example of a coherent optical detector. Further, the optical
receiver 30 may include four of optic-to-electric converters (OE)
35, four of analog-to-digital converters (ADCs) 36, and a digital
signal processor (DSP) 37.
The local oscillator light source 31 is an example of a light
source that outputs local oscillator light used for detecting light
in each of the 90-degrees hybrids 34. A laser light source such as
a distributed feedback (DFB) laser is applicable to the local
oscillator light source 31.
The PBS 32 splits the local oscillator light input from the local
oscillator light source 31 into two orthogonal polarization
components. One polarization component is input to one of the
90-degrees hybrids 34 and the other polarization component is input
to the other of the 90-degrees hybrids 34.
The PBS 33 splits polarization multiplexed signal light, which is
transmitted by the optical transmitter 10 and is received through
the optical transmission line 50, into two orthogonal polarization
components. The two polarization components are respectively input
to the 90-degrees hybrids 34 to which the local oscillator light of
the corresponding polarization components is input.
Each of the 90-degrees hybrids 34 detects polarization component
input from the PBS 33 with the local oscillator light input from
the PBS 32 to output detection light of the I component and
detection light of the Q component as a result of the
detection.
For example, one of the 90-degrees hybrids 34 detects signal light
received from the optical transmission line 50 with local
oscillator light of an orthogonal component of the polarization X
(hereinafter, also referred to as an "X polarization component") to
output detection light of the I component and the Q component.
Similarly, the other of the 90-degrees hybrids 34 detects signal
light received from the optical transmission line 50 with a
component of the other polarization Y (hereinafter, also referred
to as a "Y polarization component") to output detection light of
the I component and the Q component.
Each of the four optic-to-electric converter 35 converts the
detection light input from the 90-degrees hybrid 34 into an
electrical signal. For example, among the four optic-to-electric
converters 35, two of the optic-to-electric converters 35
correspond to the X polarization component, and the other two
optic-to-electric converters 35 correspond to the Y polarization
component.
The two of the optic-to-electric converters 35 corresponding to the
X polarization component respectively converts the I component and
the Q component of the detection light corresponding to the X
polarization component into electrical signals. Meanwhile, the two
of the optic-to-electric converters 35 corresponding to the Y
polarization component respectively converts the I component and
the Q component of the detection light corresponding to the Y
polarization component into electrical signals.
Each of the ADCs 36 converts an analog electrical signal input from
the corresponding optic-to-electric converter 35 into a digital
signal. An AC coupling element such as a capacitor may be provided
between the optic-to-electric converter 35 and the ADC 36.
The digital signal processor 37 performs digital signal processing
on digital signals of the I component and the Q component, which
are input from the ADCs 36 and are detection results of the X
polarization component and the Y polarization component.
For example, the digital signal processor 37 splits transmission
data modulated and transmitted for each of the polarizations X and
Y by the IQ modulators 15X and 15Y in the optical transmitter 10 by
using input digital signals to regenerate (or demodulate) data
transmitted by the optical transmitter 10 as reception data. The
digital signal processor 37 may be achieved by using an FPGA, an
LSI, or the like.
(PDL Loss)
Next, with reference to FIGS. 2 and 3, the influence of a
polarization dependence loss (PDL) on the polarization multiplexed
signal light transmitted in the optical communication system 1 as
illustrated is FIG. 1 will be described.
In a case where polarization multiplexed signal light is
transmitted through a medium with a PDL (hereinafter, it may be
referred to as a "PDL medium") such as the optical transmission
line 50, an optical amplifier, or the like, a difference in power
between polarization components of the signal light may be occurred
depending on a polarization state at the time when the signal light
is input to the PDL medium.
For example, as schematically illustrated in FIG. 2A, it is assumed
that polarization multiplexed signal light is transmitted between
the optical transmitter 10 and the optical receiver 30 through a
PDL medium such as the optical transmission line 50 or the optical
amplifier 70 over a plurality of stages (or spans).
In such a case, for example, as schematically illustrated in FIG.
2B, when polarization multiplexed signal light is input to the PDL
medium in a state where two orthogonal polarization principle axes
of the PDL medium coincide with the axes of the polarization
components of the polarization multiplexed signal light, a power
difference between the polarization components of the signal light
is occurred. FIG. 2B exemplarily illustrates that the power of the
Y polarization component of the signal light is degraded more than
that of the X polarization component. However, the relative
degradation relation between the components may be reversed. The
polarization principle axes of the PDL medium may be considered as
axes at which a difference between losses of the polarizations of
the PDL medium becomes the maximum.
On the other hand, as schematically illustrated in FIG. 2C, in a
case where polarization multiplexed signal light is input to the
PDL medium in a state where the axes of the polarization components
of the polarization multiplexed signal light is deviated from the
polarization principle axes of the PDL medium by 45 degrees, a
difference in power between the polarization components becomes the
minimum.
FIG. 3 is a diagram that illustrates an example of a change in a Q
penalty with respect to an input polarization state (angle) for the
PDL medium. FIG. 3 illustrates an example of a change in the
degradation (Q penalty) of the signal quality (Q value) of a case
where a total PDL of a plurality of PDL media through which
transmitted polarization multiplexed signal light passes is assumed
to be 6 dB.
As illustrated in FIG. 3, when the principle axes of the PDL medium
coincide with the axes of the polarization components of the
polarization multiplexed signal light, in other words, in a case
where the deviation angle of the input polarization wave with
respect to the principle axes illustrated in FIG. 2B is zero, the Q
penalty is degraded by about 2.7 dB. Here, the deviation angle of
the input polarization wave with respect to the principle axes of
the PDL medium may be referred to as an "input polarization angle".
As the input polarization angle increases from zero degree, the Q
penalty decreases, and the Q penalty becomes the minimum (for
example, a degradation of 1.2 dB) when the deviation angle is 45
degrees. In other words, by controlling the polarization state of
the polarization multiplexed signal light to change the input
polarization angle from zero degree to 45 degrees, the Q penalty
can be improved by about 1.5 dB.
As described above, depending on the input polarization state of
the polarization multiplexed signal light for the PDL medium, in
other words, depending on the polarization state of the
transmission signal light, a difference between the transmission
characteristic (for example, a penalty) of signal light between the
polarization components (hereinafter, may also be referred to as
"polarization channels") may be occurred.
Such transmission characteristic difference is also discussed in O.
Vassilieva et. al, "Impact of Polarization Dependent Loss and
Cross-Phase Modulation on Polarization Multiplexed DQPSK Signals",
OFC/NFOEC 2008, paper OThU6, 2008, for example.
When a difference in power between the polarization channels is
occurred, a difference in signal quality (for example,
signal-to-noise ratios (SNR)) between the polarization channels may
also occurs. Accordingly, the reception signal quality in the
optical receiver 30 may be degraded markedly because the signal
quality of one polarization component does not satisfy a required
quality.
Thus, in the present embodiment, the optical receiver 30 detects
(or monitors) the transmission polarization state of the optical
transmitter 10, which has a relatively small signal quality
degradation caused by the PDL, and the optical transmitter 10 is
controlled such that the polarization multiplexed signal is
transmitted in the detected transmission polarization state.
Thereby, it is possible to suppress the degradation of the signal
quality of the polarization multiplexed signal due to the PDL.
(Exemplary Format of Transmission Signal)
In order to detect the transmission polarization state of the
optical transmitter 10, the digital signal processor 11
(hereinafter, it may be referred to as a "transmission signal
processor 11") of the optical transmitter 10 may transmit (or
insert) a detection-oriented signal between (or into) transmission
data. The detection-oriented signal may be periodically transmitted
for the transmission data. With the periodical transmission of the
detection-oriented signal, it is possible to improve the
reliability of the detection of the transmission polarization
state.
Here, the transmission data may be referred to either the "main
signal data" or the "payload data". In addition, the
detection-oriented signal may be referred to either the "monitor
signal" or the "monitor data". The transmission data is an example
of a first signal, and the detection-oriented signal is an example
of a second signal.
The detection-oriented signal may be set as a known signal between
the optical transmitter 10 and the optical receiver 30. By setting
the detection-oriented signal as the known signal, it is possible
to improve the detection accuracy of the transmission polarization
state of the optical transmitter 10 in the optical receiver 30.
For example, the detection-oriented signal may be a signal of which
the polarization state is changed (or controlled) in time with
respect to the polarization state of the transmission data. As a
non-limited example, the detection-oriented signal may be a
polarization multiplexed signal similar to the transmission data
but, as illustrated in a frame 100 illustrated in FIG. 4, may be a
signal acquired by rotating two orthogonal polarization components
of the detection-oriented signal around a center corresponding to
an intersection of axes of the two orthogonal polarization
components of the transmission data by mutually-different angles at
different time with respect to the transmission data.
In the optical receiver 30, the digital signal processor 37 a
reception characteristic (may be referred to as a signal quality)
of the detection-oriented signal of which the polarization state is
changed in time as described above for each polarization state.
Then, the optical receiver 30 detects, based on the analyzed
result, a transmission polarization state of the optical
transmitter 10 indicative of a relatively high signal quality of
the transmission data. In other words, the polarization state
indicative of a relatively high signal quality of the transmission
data corresponds to a polarization state indicative of a relatively
low signal quality degradation due to a PDL of the transmission
data.
For example, the digital signal processor 37 may detect a
polarization state indicative of the highest signal quality among
from the reception characteristics of the detection-oriented signal
in time-varied polarization states as an optimal transmission
polarization state for the optical transmitter 10. The optical
receiver 30 transmits (or feeds back) information indicative of the
optimal transmission polarization state detected by the digital
signal processor 37 (hereinafter, it may be referred to as a
"reception signal processor 37") to the optical transmitter 10.
The transmission signal processor 11 of the optical transmitter 10
is operable to control the polarization state of the transmission
data to be the optimal transmission polarization state detected by
the optical receiver 30 based on the information fed back from the
optical receiver 30. Thereby, it is possible to suppress the
degradation of the signal quality of the polarization multiplexed
signal due to the PDL.
The modulation schemes of the detection-oriented signal and the
transmission data are not limited to particular schemes. For
example, as a non-limited example, a phase shift keying scheme such
as BPSK or QPSK, a quadrature amplitude modulation scheme called
2.sup.M-QAM (here, M represents the number of multi-levels, and
M=3, 4, 5, 6, or the like), or the like is applicable to the
modulation scheme of the detection-oriented signal. In addition, as
illustrated in a frame 200 represented in FIG. 4, a polarization
multiplexing modulation scheme such as dual polarization (DP)-QPSK,
DP-8QAM, or DP-16QAM is applicable to the modulation scheme of the
transmission data. The detection-oriented signal may be modulated
by using the polarization multiplexing modulation scheme or may be
a signal with a single polarization (SP).
The detection-oriented signal may be inserted as analog data into a
signal acquired by mapping the transmission data to a symbol
(electric field information) of a complex plane (IQ plane) which
may also be referred to as a constellation. An example thereof is
illustrated in FIGS. 5 and 6.
FIG. 5 is a block diagram that illustrates an exemplary
configuration of the transmission signal processor 11, and FIG. 6
is a block diagram that illustrates an exemplary configuration of a
detection-oriented signal inserter 112 illustrated in FIG. 5.
The transmission signal processor 11 illustrated in FIG. 5
includes, for example, a constellation mapper (hereinafter, it may
be referred to as a "mapper") 111, a detection-oriented signal
inserter 112, and a transmission waveform signal processor 113.
In the exemplary configuration illustrated in FIG. 5, the
constellation mapper 111 maps the transmission data to a symbol on
the IQ plane according to the modulation scheme.
The detection-oriented signal inserter 112 inserts the
detection-oriented signal of analog data into the transmission data
that is mapped to the symbol on the IQ plane.
The transmission waveform signal processor 113 performs, for
example, a spectrum shaping process such as a Nyquist waveform
shaping process, a skew compensating process, a band compensating
process, and the like on the transmission data into which the
detection-oriented signal is inserted by the detection-oriented
signal inserter 112.
As illustrated in FIG. 6, the detection-oriented signal inserter
112 includes, for example, a detection-oriented signal generation
and insertion unit 112a, and a detection-oriented signal
polarization controller 112b.
The detection-oriented signal generation and insertion unit 112a
generates a detection-oriented signal and inserts the
detection-oriented signal into the output of the mapper 111.
The detection-oriented signal polarization controller 112b controls
the polarization state of the detection-oriented signal inserted by
the detection-oriented signal generation and insertion unit 112a
for each polarization component, thereby changing the polarization
state of the detection-oriented signal in time. As a non-limited
example, the detection-oriented signal polarization controller 112b
may control coefficients Wxx, Wxy, Wyx, and Wyy represented by the
following Formulas (1) and (2), thereby controlling the
polarization state of the detection-oriented signal.
.times..times.''.function.''.times..times..function..times..times..PHI..t-
imes..times..function..times..times..PHI..times..times..function..times..t-
imes..theta..times..times..theta..times..times..theta..times..times..theta-
. ##EQU00001##
The calculation represented by Formulas (1) and (2) above may be
achieved by using four multipliers and two adders, as illustrated
by a frame 300 represented in FIG. 6. Ex' and Ey' in Formula 1
respectively represent data (or electric field information)
indicative of electric fields of the X polarization component and
the Y polarization component of the output signal of the
detection-oriented signal generation and insertion unit 112a and
also represent, for example, electric field information of the
detection-oriented signal having the same polarization axes as the
transmission data serving as the reference. Meanwhile, .theta. and
.phi. in Formula (2) represent parameters used to control the
polarization state of the detection-oriented signal. Accordingly,
the coefficients Wxx, Wxy, Wyx, and Wyy can be controlled by
controlling .theta. and .phi..
In the above example, although the polarization state of the
detection-oriented signal is controlled by using Formulas (1) and
(2), the polarization state may be controlled by not depending on
calculation equations. For example, the polarization state control
may be achieved by time-divisionally selecting (or switching) a
pattern among a plurality of patterns having mutually-different
polarization states which are stored in advance as the
detection-oriented signal in a storage unit (not illustrated in the
figure) such as a memory provided in the transmission signal
processor 11.
(Reception Signal Processor)
FIG. 7 illustrates an exemplary configuration focusing on the
reception signal processor (digital signal processor) 37 of the
optical receiver 30 illustrated in FIG. 1. The reception signal
processor 37 illustrated in FIG. 7 includes, for example, a
polarization splitter and waveform distortion compensator 371, a
carrier frequency and phase synchronizer 372, and a
detection-oriented signal remover 373. Such units (or circuits) 371
to 373 serves as an example of a reception data regenerator that
regenerates reception data.
In addition, the reception signal processor 37 includes, for
example, a detection-oriented signal extractor 374, a polarization
splitter and waveform distortion compensator 375, a carrier
frequency and phase synchronizer 376, a detection-oriented signal
quality analyzer 377, and an optimal polarization state detector
378. Such units (or circuits) 374 to 378 serves as an example of a
polarization state monitor that detects (or monitors) an optimal
transmission polarization state of the optical transmitter 10 based
on the reception characteristic of the detection-oriented signal
that is inserted into the transmission data as described above in
the optical transmitter 10. The "polarization state monitor" may
also be referred to as a "polarization state detector".
The polarization splitter and waveform distortion compensator 371
splits a reception digital signal input from the ADC 36 for each
polarization component and compensates for the waveform distortion
of each polarization component, for example. The compensation is,
for example, compensation (or adaptive equalization) for a waveform
distortion due to polarization mode dispersion (PMD). The adaptive
equalization may be achieved by using a plurality of linear
filters. For example, by adaptively updating parameters of the
linear filters at a speed sufficiently higher than that of a
polarization variation of signal light inside an optical fiber, it
is possible to compensate for a polarization variation or a PMD
waveform distortion accompanying a high-speed change in time.
The carrier frequency and phase synchronizer 372 removes (or
cancels) a noise component from the reception digital signal of
which the waveform distortion is compensated for each polarization
component by the polarization splitter and waveform distortion
compensator 371. Further, the synchronizer 372 estimates a correct
carrier phase and synchronizes the phase of the reception digital
signal with the estimated correct carrier phase. The noise
component may include, for example, a natural spontaneous emission
(ASE) noise, a laser phase noise of the light source 12 used for
the optical transmitter 10, and the like. A feed-back method, a
feed-forward method, or the like is applicable to the estimation of
a carrier phase, for example. The feed-back method is possible to
eliminate the influence of noises by using a digital loop filter.
The feed-forward method is possible to eliminate the influence of
noises by averaging estimated phase differences detected by a phase
detector.
Between the compensator 371 and the synchronizer 372, one or a
plurality of other processors may be provided. As an example of the
other processor may be a frequency offset compensator.
The frequency offset compensator compensates (or corrects) for a
frequency offset between signal light received from the optical
transmission line 50 and local oscillator light output by the local
oscillator light source 31 (see FIG. 1). The estimation of the
frequency offset may be performed by using, for example, an
estimation method called as a m-th square method, an estimation
method called as a pre-decision based angle differential frequency
offset estimator (PADE) method. The PADE method is possible to
increase an estimation range of the frequency offset to be larger
than that of the exponentiation method.
The detection-oriented signal remover 373 removes (or cancels) a
signal component corresponding to the detection-oriented signal
from each polarization component synchronized with the carrier
phase and outputs reception data.
A non-linear distortion compensator that compensates for a
non-linear distortion of the reception digital signal may be
arranged on the front stage or the rear stage of the
detection-oriented signal remover 373.
The detection-oriented signal extractor 374 extracts a
detection-oriented signal that is included in the reception digital
signal input from the ADC 36.
The polarization splitter and waveform distortion compensator 375,
similar to the polarization splitter and waveform distortion
compensator 371, performs splitting of polarization components and
a waveform distortion compensation for each polarization component
of the detection-oriented signal extracted by the
detection-oriented signal extractor 374. Hereinafter, the
polarization splitter and waveform distortion compensators 371 and
375 may be simply abbreviated respectively as the "compensators 371
and 375".
The synchronizer 376, similar to the synchronizer 372, performs the
process of removal of a noise component, estimation of a carrier
phase, carrier phase synchronization, and the like on the
detection-oriented signal for which the waveform distortion
compensation of each polarization component has been performed by
the compensator 375.
The detection-oriented signal quality analyzer 377 analyzes the
quality of the detection-oriented signal synchronized with the
carrier phase. For example, the analysis is performed for each
polarization state (for example, for each symbol) that changes in
time. The result of the analysis for each polarization state is,
for example, stored in a storage unit (not illustrated in the
figure) such as a memory provided in the analyzer 377. However, the
storage unit may be provided outside the analyzer 377.
The analysis of the signal quality of the detection-oriented signal
may be performed by acquiring an error vector, a bit error rate
(BER), for example. When a symbol number is denoted by k (here, k
is an integer or one or more), the error vector may be acquired by
a subtraction of "a received detection-oriented signal R(k)-a
transmitted detection-oriented signal T(k)", for example. A
standard deviation of the amplitude of the error vector may be used
as the reception signal characteristic, for example.
The transmission detection-oriented signal T(k) is an example of a
known signal transmitted by the optical transmitter 10 as described
above. The signal T(k) may be stored in the storage unit that
stores the result of the analysis of the polarization state as
described above in advance. However, the storage unit storing the
transmission detection-oriented signal T(k) may be a storage unit
other than the storage unit storing the result of the analysis of
the polarization state.
The optical receiver 30 may be notified of the transmission
detection-oriented signal T(k) by the optical transmitter 10 in
advance. This notification may be performed by using, for example,
supervisory signal light that is also called as supervisory (SV)
light or optical supervisory channel (OSC) light. In other words,
the transmission detection-oriented signal T(k) may be overlapped
with (or mapped to) supervisory light that is transmitted from a
source node of an optical transmission apparatus provided with the
optical transmitter 10 to a destination node of an optical
transmission apparatus provided with the optical receiver 30.
The optimal polarization state detector 378 detects an optimal
polarization state based on each polarization state of the
transmission data and the detection-oriented signal acquired by the
compensators 371 and 375 and the analysis result (or the reception
quality of the detection-oriented signal) for each time (for
example, for each symbol) acquired by the analyzer 377. For
example, among from a plurality of relations between the
polarization state of transmission data and the polarization state
of the detection-oriented signal for each time, the detector 378
may detect a relation of the specific polarization state which
makes the reception signal quality of the detection-oriented signal
to be the best.
The detection result is, for example, transmitted (fed back) to the
optical transmitter 10 as optimal polarization state information.
The feed-back (or notification) of the optimal polarization state
information may be performed by using, for example, supervisory
signal light, OSC light, or the like. In other words, the optimal
polarization state information may be overlapped with (or mapped
to) supervisory light that is transmitted from a source node of an
optical transmission apparatus provided with the optical receiver
30 to a destination node of an optical transmission apparatus
provided with the optical transmitter 10.
The detection-oriented signal quality analyzer 377 and the optimal
polarization state detector 378 may be considered as a polarization
state detector that detects a transmission polarization state of
the optical transmitter 10, which indicates a relatively high
signal quality of the transmission data, based on the reception
characteristic of the detection-oriented signal.
Next, FIG. 8 illustrates an exemplary configuration of the optical
transmitter 10 focusing on the transmission signal processor 11
operable to control the polarization state of the transmission
optical signal based on the optimal polarization state information
detected by and notified from the optical receiver 30 as described
above. The transmission signal processor 11 illustrated in FIG. 8
includes, for example, a polarization controller 113a in the
transmission waveform signal processor 113 illustrated in FIG.
5.
The polarization controller 113a performs optimization control of
the polarization state of the transmission data based on the
optimal polarization state information notified (or fed back) from
the optical receiver 30. For example, similar to the polarization
control performed by the detection-oriented signal inserter 112
illustrated in FIG. 6 for the detection-oriented signal, the
polarization controller 113a controls the polarization state of the
transmission data to be the optimal transmission polarization state
notified from the optical receiver 30 by controlling the
coefficients Wxx, Wxy, Wyx, and Wyy represented in the
aforementioned Formula 2.
In other words, the digital signal processor 37 of the optical
receiver 30 may be considered as a control apparatus that controls
the transmission polarization state of the optical receiver 30 to
be the optimal transmission polarization state detected based on
the reception characteristic of the detection-oriented signal as
described above. This is also applied to first and second modified
examples described later.
As described above, according to the embodiment, the optical
transmitter 10 transmits an optical signal while relatively (in
time) changing the polarization state of the detection-oriented
signal with respect to the polarization state of the transmission
data. The optical receiver 30 analyzes the reception characteristic
of the detection-oriented signal, detects an optimal polarization
state of the transmission data based on the result of the analysis,
and notifies the optical transmitter 10 of the detected
information. The optical transmitter 10 performs optimization
control of the polarization state of the transmission data based on
the notified information. Thus, even when polarization multiplexed
signal light transmitted by the optical transmitter 10 is
propagated through the optical transmission line 50 or a PDL medium
such as the optical amplifier 70, it is possible to suppress (or
minimize) the degradation of the quality of the optical signal due
to a PDL.
(First Modified Example)
In the transmission signal processor 11 illustrated in FIG. 5, the
detection-oriented signal inserter 112 is provided on the rear
stage of the mapper 111. However, as illustrated in FIG. 9, the
detection-oriented signal inserter 112 may be provided on the front
stage of the mapper 111. In other words, the detection-oriented
signal may be inserted into the transmission data before being
mapped to the IQ plane in time (for example, for each symbol) as a
data pattern of which the polarization state changes.
In the configuration illustrated in FIG. 9, the detection-oriented
signal inserter 112 inserts a data pattern of which the
polarization state changes, for example, for each symbol into the
transmission data before being mapped to symbols on the IQ
plane.
The data pattern of which the polarization state changes in time
can be generated by applying Stokes-vector-modulation, for example.
Thus, the detection-oriented signal inserter 112 of this example
may be referred to as a Stokes-vector-modulator 112.
The constellation mapper 111 maps the transmission data inserted
with data patterns of which the polarization state changes for each
symbol to symbols on the IQ plane.
The transmission waveform signal processor 113 performs, for
example, a spectrum shaping process such as a Nyquist waveform
shaping process, a skew compensating process, a band compensating
process, and the like on the transmission data inserted with the
detection-oriented signal and mapped to symbols on the IQ pane by
the mapper 111.
FIG. 10 schematically illustrates an example for applying the
Stokes-vector-modulation to the detection-oriented signal. FIG. 10
schematically illustrates that a signal is mapped to electric field
information in a three dimensional space (called as a Poincare
sphere or a Stokes space) having three orthogonal axes defined by
Stokes vectors S1, S2, and S3 as illustrated inside a frame 400.
The three axes of the Poincare sphere may be respectively referred
to as an S1 axis, an S2 axis, and an S3 axis.
Two intersections between the S1 axis and the Poincare sphere
surface represent polarizations that are orthogonal to each other.
For example, an intersection on the positive side of the S1 axis
represents an X polarization, and an intersection on the negative
side of the S1 axis represents a Y polarization orthogonal to the X
polarization.
Two intersections between the S2 axis and the Poincare sphere
surface represent 45 degrees linear polarizations having
mutually-inversed signs. For example, an intersection on the
positive side of the S2 axis represents a 45 degrees linear
polarization, and an intersection on the negative side of the S2
axis represents a -45 degrees linear polarization.
Two intersections between the S3 axis and the Poincare sphere
surface represent circular polarizations having mutually-inversed
rotation directions. For example, an intersection on the positive
side of the S3 axis represents a clockwise circular polarization,
and an intersection on the negative side of the S3 axis represents
a counterclockwise circular polarization.
The transmission data may be mapped, for example, to electric field
information (or symbols) corresponding to the two intersections
between the S1 axis and the surface of the Poincare sphere with the
S1 axis being set as the main axis of the polarization. Meanwhile,
the detection-oriented signal may be mapped to electric field
information corresponding to an arbitrary polarization represented
in the Poincare sphere. The mapping pattern may be a uniform
pattern or a non-uniform pattern in the Poincare sphere.
In the example illustrated by (A) inside the frame 400 depicted in
FIG. 10, the detection-oriented signal may be mapped to any of the
following first to three intersection groups such that mapped
patterns are different from each other in time. The first
intersection group may correspond to the two intersections between
the S1 axis and the surface of the Poincare sphere. The second
intersection group may correspond to the two intersections between
the S2 axis and the surface of the Poincare sphere. The third
intersection group may correspond to the two intersections between
the S3 axis and the surface of the Poincare sphere.
In the example illustrated by (B) inside the frame 400 depicted in
FIG. 10, the detection-oriented signal may be mapped to electric
field information corresponding to two points opposed to each other
with respect to the center of a circle corresponding to the equator
of the Poincare sphere such that mapped patterns are different from
each other in time. In this case, symbols mapped with the
detection-oriented signal may be arranged uniformly on the circle
corresponding to the equator of the Poincare sphere.
In the example illustrated by (C) inside the frame 400 depicted in
FIG. 10, the detection-oriented signal may be mapped to electric
field information corresponding to any one of apexes of a
rectangular parallelepiped illustrated by dotted lines in the
Poincare sphere such that mapped patterns are different from each
other in time.
Since the detection-oriented signal is used to detect an optimal
polarization state of the transmission data, the detection-oriented
signal is not necessary to be in a format capable of optimizing
data rate.
The Stokes-vector-modulation as described above makes the
polarization state of the detection-oriented signal possible to be
different in time (for example, for each symbol) independently from
the transmission data. Thereby, it is possible to reduce the
influence of the transmission data on the reception quality
characteristic. In addition, since the Stokes vector of an optical
signal is determined by a phase difference and an amplitude ratio
of two orthogonal polarizations, the Stokes vector does not depend
on the absolute phase of light.
In other words, the absolute phase of the carrier is not used for
the Stokes-vector-modulation. Thus, the process performed in the
optical receiver 30 can be simplified, and accordingly, the load of
the reception signal processing can be reduced. For example, as
illustrated in FIG. 11, the above-described carrier frequency and
phase synchronizer 376 (see FIG. 7) for the detection-oriented
signal is not necessary in the optical receiver 30.
FIG. 11 is a block diagram that illustrates an exemplary
configuration of the reception signal processor 37 used for the
optical receiver 30 for the optical transmitter 10 that performs
the Stokes-vector-modulation.
As illustrated in FIG. 11, the reception signal processor 37 for
the detection-oriented signal is different in that a Stokes vector
calculator 376A, a symbol-based characteristic analyzer 377A, and
an optimal polarization state detector 378A are provided in
comparison with the configuration illustrated in FIG. 7. Such units
(or circuits) 376A to 378A may be considered as the aforementioned
polarization state detector.
When electric field information corresponding to components of the
X polarization and the Y polarization, for which the wave
distortion has been compensated by the compensator 375, is denoted
by Ex and Ey, the Stokes vector calculator 376A calculates Stokes
vectors S1 to S3 represented by the following Formulas (3) to (5).
Here, .delta. in Formulas (4) and (5) is represented by Formula
(6). S1=|Ex|.sup.2-|Ey|.sup.2 (3) S2=2|Ex||Ey|cos.delta. (4)
S3=2|Ex||Ey|sin.delta. (5) .delta.=arg(Ey/Ex) (6)
The symbol-based characteristic analyzer 377A analyzes a signal
quality (or reception characteristic) of the detection-oriented
signal for each symbol based on the Stokes vectors S1 to S3
calculated by the Stokes vector calculator 376A.
The analysis of the signal quality of the detection-oriented signal
may be performed, for example, by acquiring an error vector, a BER,
or the like. When a symbol number is denoted by k (here, k is an
integer or one or more), the error vector may be acquired by a
subtraction of "a received detection-oriented signal R(k)-a
transmitted detection-oriented signal T(k)". For example, a
standard deviation of the amplitude of the error vector may be used
as the reception signal characteristic.
The optimal polarization state detector 378A may detect a
polarization state in which the reception characteristic of the
detection-oriented signal for each symbol is the best based on the
analyzed result (the reception quality of the detection-oriented
signal) acquired by the analyzer 377A for each symbol. The
information indicative of the detected polarization state, similar
to the embodiment described above, may be fed back (or notified) to
the optical transmitter 10 as optimal polarization state
information.
In a frame 500 illustrated in FIG. 11, an example is illustrated in
which it is optimal to map the transmission data to electric field
information corresponding to two intersections between the S2 axis
the Poincare sphere when the Stokes-vector-modulation of the
pattern (A) illustrated in FIG. 10 (frame 400) is applied to the
detection-oriented signal.
In the embodiment and the first modified example described above,
the patterns of the detection-oriented signal is changed for each
symbol. However, the patterns of the detection-oriented signal may
be changed in any other unit such as a frame unit. In such a case,
the optical receiver 30 may detect an optimal transmission
polarization state for the optical transmitter 10 by detecting a
reception characteristic of the detection-oriented signal in the
unit for the change.
In the configuration illustrated in FIG. 11, the polarization
splitter and waveform distortion compensators 371 and 375 are
respectively provided for the transmission data and the
detection-oriented signal. In other words, the polarization
splitting and the waveform distortion compensation are performed
for each of the transmission data and the detection-oriented
signal.
However, when the Stokes-vector-modulation is applied as described
above, since the main axis (for example, the S1 axis) of the
polarization of the transmission data can be locked by the
polarization splitting performed on the reception side, it is
possible to easily identify the mapped pattern illustrated in the
frame 500 in FIG. 11.
Accordingly, as illustrated in FIG. 12, the polarization splitter
and waveform distortion compensators 371 and 375 illustrated in
FIG. 11 may be replaced by a polarization splitter and waveform
distortion compensator 371a common to the transmission data and the
detection-oriented signal. Since the circuit scale of the
"polarization splitter and waveform distortion compensator" may
easily increase compared to the other blocks, the above common
configuration makes possible the reception signal processor 37 to
be further simplified and low-cost.
(Second Modified Example)
As illustrated in FIG. 13, mutually-different modulation schemes
are applicable to the detection-oriented signal and the
transmission data. As a non-limited example, the
Stokes-vector-modulation may be applied to the detection-oriented
signal, and DP-QPSK modulation may be applied to the transmission
data.
In such a case, a guard interval may be arranged such that the
transmission data is not subjected to a waveform distortion or the
like by the detection-oriented signal. In the guard interval, for
example, a dummy signal modulated by using the same modulation
scheme (in the example illustrated in FIG. 13, the DP-QPSK) as that
of the transmission data may be inserted (transmitted). The
insertion of the dummy signal may be performed by the
aforementioned detection-oriented signal inserter 112, for
example.
Thereby, even when mutually-different modulation schemes are
applied to the detection-oriented signal and the transmission data,
it is possible to suppress the signal quality degradation of the
transmission data.
(Others)
In the embodiment and the modified examples described above, the
transmission period (or cycle) of the detection-oriented signal may
be changed according to a transmission parameter such as a
characteristic of the optical transmission line 50, for example. In
addition, in the transmission signal format illustrated in FIGS. 4,
10, and 13, the occupancy ratio of the detection-oriented signal to
the transmission data is not limited to a particular ratio. Here,
as the occupancy ratio of the detection-oriented signal increases,
the detection accuracy of the optical transmitter 10 for an optimal
polarization state is improved; however, the transmission capacity
of the transmission data decreases. Thus, there is a trade-off
between the detection accuracy and the transmission capacity and
the occupancy ratio may be optimized as is necessary.
According to the technology described above, it is possible to
detect a specific polarization state available to suppress the
degradation of the signal quality due to a polarization dependence
loss of an optical signal.
All examples and conditional language provided herein are intended
for pedagogical purposes to aiding the reader in understanding the
invention and the concepts contributed by the inventor to further
the art, and are not to be construed as limitations to such
specifically recited examples and conditions, nor does the
organization of such examples in the specification relate to a
showing of the superiority and inferiority of the invention.
Although one or more embodiment(s) of the present invention have
been described in detail, it should be understood that the various
changes, substitutions, and alterations could be made hereto
without departing from the spirit and scope of the invention.
* * * * *
References